Semiconductor manufacturers strive to reduce the size of devices in order to reduce power consumption and increase gate density and thereby deliver faster circuits with greater processing power at lower cost to their customers. This has led to development of generations of photolithographic equipment for patterning the semiconductor wafers where the minimum feature size, e.g., the width of the smallest patterned line, has shrunk with each successive generation. Most high-resolution equipment currently in use can produce 14 nm feature sizes with the industry planning to move to smaller feature sizes in the near future.
Current practice typically uses UV light at 193 nm wavelength when patterning the semiconductor wafer. In order to produce features significantly smaller than the wavelength, immersion lithography is used to reduce the wavelength below by forming the image in a liquid with refractive index >1, as well as using multiple patterning to produce features smaller than the spot width. It is generally accepted that moving to smaller features will require extreme ultraviolet (EUV) lithography. While EUV typically refers to wavelengths between 120 nm and 10 nm, it is generally expected that this next-generation lithography will use 13.5 nm light emitted by laser-produced plasma sources with CO2 lasers exciting tin droplets. Future generations may require soft x-ray (SXR) lithography with wavelengths <10 nm.
In order to efficiently and economically produce good parts, the masks used in the photolithographic process must be nearly free of defects. This typically requires inspection of the masks at multiple points in the masks life cycle. The blanks must be inspected before and after the mask pattern is produced, when a potential defect has been identified in a mask in use for lithography, and/or after a defect has been repaired.
The masks used for semiconductor photolithography are oversized with the mask image demagnified at the surface of the wafer, typically by 4×. Thus, a 22 nm feature is 88 nm at the mask and a 14 nm feature is 56 nm at the mask. This dimension is smaller than the Abbe resolution limit, related to the inspection wavelength, of currently developed optical inspection tools that generally use wavelengths of 193 nm and longer.
A variety of approaches have been developed to inspect the masks and mask blanks. Typically, these approaches must image the mask with a resolution comparable to or smaller than the feature size of the process in order to reliably spot significant defects in the mask. The challenge is to do so with an affordable instrument which can reliably identify and analyze defects significant enough to affect the lithography process.
One approach for inspecting the masks or mask blanks uses synchrotron radiation as the illuminating light source. Using relatively bright synchrotron light (see K. A. Goldberg, I. Mochi, “Wavelength-Specific Reflections: A Decade of EUV Actinic Mask Inspection Research,” J. Vac. Sci. Technol. B 28 (6), C6E1-10 (2010). DOI 10.1116/1.3498757), which has an arbitrarily selectable wavelength, EUV microscopes can image significant areas of EUV masks with resolution down to 22 nm, with current technology (see Markus P. Benk, Kenneth A. Goldberg, Antoine Wojdyla, Christopher N. Anderson, Farhad Salmassi, Patrick P. Naulleau, and Michael Kocsis, “Demonstration of 22-nm half pitch resolution on the SHARP EUV microscope,” Journal of Vacuum Science and Technology B33 (6), 06FE01 (2015). DOI 10.1116/1.4929509). However, synchrotrons are large accelerators that are quite expensive and require large, dedicated facilities, which imposes significant practical limitations, i.e., cost and availability, on their utility for mask inspection. In addition, these synchrotron inspection systems have not yet achieved finer resolution for routine imaging, so there is room for improvement using different imaging techniques.
It is far more desirable to perform EUV mask inspection with a smaller instrument that can be installed in a semiconductor fabrication facility. Such an instrument would have lower cost and much higher availability than large, dedicated synchrotron facilities and would integrate into the workflow of the fabrication facility. KLA-Tencor described a concept for an EUV mask inspection platform labeled the 710 System (see “Solutions for EUV Mask and Blank Inspections 2012 D. Wackand G. Inderhees.pdf”). This proposed system (designated the 7xx tool) has a module located inside the cleanroom with dimensions 9 m×3 m×3.1 m (length×width×height) and 2 modules in the room below, outside the cleanroom, each with comparable footprints and volumes as the module inside the cleanroom. Such a system is small enough to be installed as one of the suite of instruments present in the cleanroom of a semiconductor fabrication facility and hence be used in the normal workflow.
Currently proposed systems for EUV mask inspection typically use one of three approaches to inspect masks with the highest resolution: (1) inspection is done optically with UV light of 193 nm or longer, which has a short enough wavelength to detect many features but can still transmit through air and some optical materials like fused silica; (2) inspection is done with scanning electron or ion beams, as in a scanning electron microscope (SEM); and (3) inspection is done with a scanning tunneling microscope, such as an atomic force microscope (AFM).
Each of these approaches has some practical drawbacks. Approach (1) is limited in resolution by the wavelength of the light used. Approaches (2) and (3) are limited by the scanning rate of the microscope and hence the throughput they can achieve. Approach (2) is also limited logistically by the vacuum environment it requires. Approach (3) is also limited in that the tip of the scanning tunneling microscope cannot penetrate the surface of the object, unlike approaches that use radiation which may penetrate to some depth and hence can provide some information about the material underlying the surface or its three dimensional composition.
However, the greatest drawback to these approaches is they do not directly measure the characteristic that can predict the pattern the mask will project when used for lithography, namely, spatially varying reflectivity of the mask at the relevant wavelength. Approach (1) senses the reflectivity at a different wavelength. Approaches (2) and (3) do not image the reflectivity of electromagnetic radiation; instead, they measure interaction with a charged particle or tunneling to a very sharp tip.
Industry experts generally agree that EUV lithography will require actinic inspection, i.e., inspection done with the same wavelength as when performing the photolithography. This expectation stems from two facts: (1) the resolution of an optical system scales with the operating wavelength as R=kλ/NA, where k is a process factor, λ is the wavelength, and NA is the numerical aperture of the objective lens, so a shorter wavelength can resolve smaller features; and (2) the effect of a defect is best gauged by its effect on light of a wavelength comparable to what will be used during the photolithography process.
The second point is especially important since some mask defects are primarily phase defects, i.e., they change the phase of the light projected onto the wafer. For EUV lithography, the mask, like all of the optics, is reflective, since transmission losses are too large for practical materials. The mask is formed by applying a patterned reflective coating to a substrate. At EUV wavelengths, reflective coatings are Bragg reflectors, a stack of thin film coatings designed for high reflectivity, produced in a manner similar to multilayer dielectric coatings for visible and near-visible wavelengths. The coatings are typically multilayer stacks of silicon and molybdenum with tens of layers.
If there is a defect or particle on the substrate, it is possible that the multilayer coating will be reflective, but will have a phase shift due to the local surface being higher or lower than the surrounding surface. A similar effect can occur with a particle or defect that is embedded in the multilayer coating. Due to the stringent resolution requirements and partial coherence of the EUV illumination used during lithography, these phase shift errors can significantly affect the geometry or the sharpness of the geometry patterned by the lithography. However, these phase shift errors may not be detectable using non-actinic radiation for inspection, e.g., longer wavelength light (193 nm) or the electron beam of a scanning electron microscope (SEM). More generally, a possible defect may affect a non-actinic radiation beam much differently than the actinic beam. Hence inspection with actinic radiation is preferred to inspection with non-actinic radiation, in order to better predict the defect's affect on the photolithography, along with the ability to detect phase defects.
One approach to this problem would be to use conventional EUV sources, similar to those in development or in use for EUV lithography, together with conventional optical systems similar to those used with 193 nm inspection tools, except for their use of reflective optics. These conventional EUV sources create a hot plasma which then radiates at ˜13.5 nm, either by using powerful CO2 lasers to turn tin droplets into plasma or by some other means of creating and exciting the plasma. For EUV wavelengths, the optical system consists of reflective optics whose surface figures (a term of art referring to the accuracy with which an optical surface conforms to its intended shape) are accurate enough for 13.5 nm light, over an order of magnitude more accurate than optics for 193 nm tools.
Such a system has many drawbacks. The optical system is expensive due to its complexity and exacting specifications. The plasma sources typically produce ions which erode the optical surfaces over time. The optical system performs incoherent imaging which is not very sensitive to phase defects. And the very short wavelengths require that the focal distance, i.e. the distance from the mask surface to the optics, be controlled with extremely tight tolerances, which can be very challenging when scanning across a mask that is ˜10 cm across.
Such systems have been produced, for example the EUV AIMS system produced by Zeiss (see Garetto et al., Advanced Optical Technologies. Volume 1, Issue 4, Pages 289-298, ISSN (Online) 2192-8584, ISSN (Print) 2192-8576, DOI: https://doi.org/10.1515/aot-2012-0124, September 2012). However, due to the drawbacks mentioned, these systems are not capable of scanning an entire mask for comprehensive inspection. Instead, they are used exclusively for limited imaging of small regions of a mask, often for a task titled “defect review” where a defect has already been identified, perhaps because a test lithography run yielded a device which did not perform properly when tested electrically.
There exists a need for an actinic mask inspection system for EUV lithography that can examine masks for defects using a comparable wavelength to the one that will be used for photolithography. The system should be able to image phase defects as well as amplitude defects, and scan masks with adequate throughput to inspect the entire mask in a reasonable period of time. The system should also be significantly less complicated than an approach that requires mirrors whose surface figures and alignment must be accurate to several nanometers while scanning across the surface of the mask.
One approach others have taken is a coherent imaging approach capable of imaging phase and amplitude defects. Termed coherent diffractive imaging (CDI), the object is illuminated with a coherent beam. An imaging sensor records an image of the intensity of the diffraction pattern (see M. Seaberg, et al., “Ultrahigh 22 nm resolution coherent diffractive imaging using a desktop 13 nm harmonic source,” Opt. Exp. 19, 22470 (2011)). Since much of the light is transmitted or reflected in the 0th order, it must be blocked with a beam stop along the optical axis so as not to wash out the off-axis diffraction pattern. This method uses iterative phase retrieval algorithms, typically with some finite support constraints, to estimate the phase and amplitude object from the diffraction intensity.
While CDI does yield relatively high resolution images, it is an inefficient use of the beam energy, since much of the beam is lost in the 0th order. It also requires many iterations, hence lengthy computations, to produce the output. The number of iterations required to obtain a high quality reconstruction is not predictable and may take tens or hundreds of iterations. This computation, along with the inefficient use of the illuminating beam, represent serious limitations to using CDI for inspection tasks. It would be much more advantageous to use an approach that produces deterministic imagery which can be produced in real time.
In the following specification, we shall use conventional optical imaging nomenclature to refer to the target of the system as the object. Our discussion centers on application to the inspection and imaging of reflective EUV semiconductor lithography masks. However, it will be appreciated that with modifications obvious to those skilled in the art, the present invention has general application to imaging other objects with comparable resolution, including objects imaged in transmission.